On Linux/MacOS/Android devices on the same local network, there are no such problems. Interestingly, this problem only occurs on Windows devices.
Create a DWORD parameter with the name AssumeUDPEncapsulationContextOnSendRule and the value 2. Windows XP/Windows Server 2003 - HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\IPSec. Windows 10/8.1/Vista and Windows Server 2016/2012R2/2008R2 - HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\PolicyAgent. 
Open the Registry Editor ( regedit.exe) and go to the following registry key:. If the L2TP/IPsec VPN server is behind a NAT device, in order to connect external clients through NAT correctly, you have to make some changes to the registry both on the server and client side to allow UDP packet encapsulation for L2TP and NAT-T support in IPsec. NAT-T is enabled by default in almost all operating systems (iOS, Android, Linux) except Windows. You can fix this drawback by enabling support for the NAT-T protocol, which allows you to encapsulate ESP 50 packets in UDP packets on port 4500. If you want to use IPSec for communication, Microsoft recommends using public IP addresses on the VPN server.īut there is also a workaround. This is because IPsec uses ESP (Encapsulating Security Payload) to encrypt packets, and ESP doesn’t support PAT (Port Address Translation). 
The Windows built-in VPN client doesn’t support by default L2TP/IPsec connections through NAT. VPN Error 809 for L2TP/IPSec on Windows behind NATĪs it turned out, the problem is already known and described in the article. If you connect to the same VPN server via PPTP, the connection is successfully established.
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